1) Field of the Invention
The present invention relates to a method of inputting signals to a light modulator and a simultaneous multi-beam type light modulating apparatus, and particularly to a method of mixing together a plurality of inputted signals whose frequencies are different from each other and whose frequency intervals are equal to each other so as to input the thus-mixed signals at a time to a light modulator for dividing incident light beams with the intensity or level according to the amplitude of each of the inputted signals and in the direction corresponding to the frequency of each of the inputted signals so as to emit the thus-divided light beams, and to a simultaneous multi-beam type light modulating apparatus using the signal inputting method referred to above.
2) Description of the Related Art
There has heretofore been known a light modulating apparatus used with a multi-frequency acousto-optic device (hereinafter called merely "AOM") (see Japanese Patent Application Publication No. 63-5741, Japanese Patent Application Laid-Open No. 54-5455, Japanese Patent Application Laid-Open No. 57-41618, and Japanese Patent Application Publication No. 53-9856). A plurality of high-frequency signals whose frequencies differ from each other, which are produced from a plurality of oscillation circuits, are mixed together and the thus-mixed signals are inputted to the AOM used for the light modulating apparatus at a time. Thus, the light modulating apparatus has the problem that since the plurality of high-frequency signals are mixed together, harmonic signals are produced, and hence non-uniformity of the density of an image is produced under the influence of harmonic distortion (two-signal third-order distortion) due to a third-order harmonic signal in particular. Let's now consider a case where eight high-frequency signals are mixed two by two by way of example, and the thus-mixed signals are further mixed together two by two, followed by further mixing of the signals thus mixed, thereby inputting the mixture to the AOM. This method will be described as follows.
Let's now assume that the intervals between adjacent frequencies of high-frequency signals generated from respective oscillation circuits are set equal to each other and their frequencies are represented by f.sub.1, f.sub.2, . . ., f.sub.8 (for example, 110 MHz, 120 MHz, . . ., 180 MHz). When the high-frequency signals represented in the form of the frequencies f.sub.1 and f.sub.2 are mixed together, third-order harmonic signals which provide the maximum intermodulation interference are produced at positions where frequencies 2f.sub.1 -f.sub.2, 2f.sub.2 -f.sub.1 are present as shown in FIG. 7(1). Similarly, when the high-frequency signals represented in the form of the frequencies f.sub.3, f.sub.4 are mixed together, third-order harmonic signals are produced at positions where frequencies 2f.sub.3 -f.sub.4, 2f.sub.4 -f.sub.3 are present as shown in FIG. 7(2). Thus, if all the signals represented by the frequencies f.sub.1, f.sub.2, . . ., f.sub.8 are mixed together after the signals whose frequencies are adjacent to each other are mixed, third-order harmonic signals are produced at positions where the frequencies 2f.sub.1 -f.sub.2, f.sub.1, f.sub.2, . . ., f.sub.8, 2f.sub.8 -f.sub.7 are present as shown in FIG. 7(3). As a consequence, the signals generated from the respective oscillation circuits are distorted and high-leveled third-order harmonic signals are produced at the positions where the frequencies 2f.sub.1 -f.sub.2, 2f.sub.8 -f.sub.7 are present. Thus, even when the signal represented by the frequency f.sub.4 is set to a spaced (OFF) state by way of example, a third-order harmonic signal is produced at the position of the presence of the frequency f.sub.4 as shown in FIG. 7(4). In addition, the level of each of the high-frequency signals at the positions where the frequencies f.sub.3 and f.sub.6 are present is reduced due to the generation of the third-order harmonic signal. Consequently, the laser beams cannot be divided according to the duration (ON) and the space (OFF) of the image data with the AOM, thereby causing the nonuniformity of the density of an image.
When the number of times in which the high-frequency signals are mixed is increased, the intensity or degree of modulation of light according to each of the frequencies is reduced under the influence of high-frequency signals represented by other frequencies. Thus, an approach for controlling the level of each of high-frequency signals mixed together by making use of an AGS (Automatic Gain Control), i.e., the amplitude of each of the high-frequency signals, or for controlling the level of each of the high-frequency signals mixed together by making use of a logic circuit and an attenuator has heretofore been performed to avoid variations in the degree of modulation of the light according to each frequency under the influence of high-frequency signals represented by other frequencies.
However, the above-described conventional light modulating apparatus simply controls the level of each of the already-mixed high-frequency signals. Therefore, the level, i.e., amplitude of each of the high-frequency signals generated from the oscillation circuits can be adjusted separately, and the accuracy in oscillations of each oscillation circuit varies due to errors in setting frequencies caused upon manufacture, so that the level of each of all the high-frequency signals cannot be rendered constant Thus, the conventional light modulating apparatus has the problem that the degrees of modulation of the light differ from each other owing to the difference in amplitude between outputs from the oscillation circuits, which is caused by variations in the accuracy of the oscillations of each oscillation circuit at the time that it is employed in recording of an image, thereby causing the nonuniformity of the density of the recorded image.